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Oxygen is supposed to have driven the evolution of complex life – but the discovery of animals that thrive without it tells a different story
Living without breathing
Some fish, mussels and sediment-dwelling worms can live without oxygen for hours or even days. Instead of getting energy by "burning" food, the cells of these animals switch to ways of producing energy that do not require oxygen. Until earlier this year, no animals had been discovered that go their entire lives without oxygen (see main story) - it was thought to be impossible.
Oxygen is not only used for getting energy from food, it is also needed to make compounds like collagen, the "glue" that holds animals together. No oxygen, no collagen; no collagen, no animals, the thinking went. That must be wrong, although we have yet to work out how the newly discovered animals make compounds like collagen without oxygen.
So could there be planets out there with large animals that do not need oxygen? While burning food produces 10 times as much energy as other means like fermentation, in theory an animal might get around that if it could somehow get 10 times as much fuel. The trouble is, fermentation leaves far less energy for predators in ecosystems. With aerobic respiration, there can be five or six links in a food chain before the amount of energy falls below 1 per cent of that available initially. Without oxygen, this happens with just two links.
And with far less scope for predation, animals might not evolve as far or as fast; the need to find prey or dodge predators is thought to have driven the development of features like eyes and mouths and muscles.
The ability of ancestral mitochondria to make hydrogen, rather than use oxygen, was the basis of the primordial pact that gave rise to the eukaryotes, Martin and Müller argued. The bacteria produced hydrogen as waste, and the host cell used it to convert carbon dioxide into methane, gleaning a little energy from the process - just as many archaea, called methanogens, still do. The symbiosis began in an environment with little or no oxygen and only later, after the relationship was well established, did the host cell start exploiting the ability of the ancestral mitochondria to use oxygen.
This idea, known as the "hydrogen hypothesis", was proposed by Martin and Müller in 1998 (Nature, vol 392, p 37), but it has never gained widespread acceptance. It was not just up against the gut feeling of most researchers that the rise of the eukaryotes was related in some way to oxygen; on the face of it, what little evidence there was did not support it either.
Species name: Not yet assigned, but of the phylum Loricifera, genus Spinoloricus
Habitat: Deep sediments lacking oxygen in the L'Atalante basin of the Mediterranean Sea south of Greece – and who knows where else…
Until earlier this year, no animals had been discovered that go their entire lives without oxygen (see main story) - it was thought to be impossible.
NOBODY likes to admit they are wrong. It doesn't matter if it is a scientist, theologian or some random person on the street.
. At the base of their tubes, hydrothermal fluid is enriched in H2S and CO2, but is devoid of oxygen. The respiratory plume is extended into the ambient (2°C), oxygen-enriched bottom water. Riftia's unusual microhabitat is the interface between the hydrothermal fluids and the ambient bottom water where essential metabolites can be taken up by the plume and transported to internal bacteria for metabolism. The steep thermal and chemical gradients provide access to the reduced compounds needed to fuel growth and the oxygen needed to burn the fuel.
Vesicomyid clams living in hydrothermal vents have endosymbiont-containing gills. Vesicomyid blood transports oxygen bound to hemoglobin and contains an extracellular component with a high sulfide binding affinity (18). In this species, sulfide and oxygen acquisition are spatially separated. The foot of the clam is extended down into crevices that vent sulfide-rich water, enabling sulfide uptake and transport through the circulatory system to gills that are bathed in oxygen-rich seawater circulated from above.
You are correct about one thing though, through evolution, life will always find a way.
Evolutionary biologists document the fact that evolution occurs, and also develop and test theories that explain its causes.
Cellular respiration (process that uses oxygen to break sugar into energy
for life functions) and photosynthesis (green plants converting sunlight
energy into sugars using water and carbon dioxide) are distinct and
separate processes. Animals only use cellular respiration. Plants do not
have muscles and the other functions necessary in animals functioning that
require a high levels of oxygen. Plants also CAN NOT take the energy
directly for their energy needs from photosynthesis. Photosynthesis
produces a far greater amount of oxygen and sugars, etc. then the small
amount the plant requires to produce its energy needs. OK - but only
during the day when the sun light is available! During the night, the
plant actually uses oxygen it has left over from the daylight
photosynthesis or takes the oxygen from the air surrounding the plant to
meet its energy needs. This is not nearly the amount of oxygen an animal
needs over the same period of time.